The present invention relates generally to the field of digital communications and more specifically to modulation and spread spectrum technologies. Spread spectrum techniques have proven to be extremely effective in building multiple access systems, combating or suppressing jamming and interference, transmitting signals at low power, and achieving message privacy from undesired listeners. Spread spectrum technology is characterized by a signal occupying an allocated bandwidth that is much wider than the data rate at which information is communicated. While many of the benefits and fundamental principals of spread spectrum techniques are well-known, actual design of spread spectrum systems can vary widely and face significant challenges in terms of performance, cost, and other considerations.
Spread spectrum systems commonly involve Phase Shift Keying (PSK) modulation as the modulation technique used in transmitting spread spectrum signals. PSK modulation refers to a general category of modulation schemes that may includes Binary Phase Shift Keying (BPSK), Quadrature Phase Shift Keying (QPSK), and others. PSK modulation schemes are often associated with “spill-over” of signal energy from the allocated bandwidth into adjacent bandwidths. This effect may be reduced by incorporating various envelope, waveform shaping, and/or other techniques. However, the need to improve spectral efficiency of modulated signals remains a fundamental problem in spread spectrum systems. This is especially true in limited bandwidth environments such as wireless and satellite systems, where neighboring signals packed closely together in the frequency spectrum may experience significant interference from one another due to unwanted “spill-over” of each signal outside of its allocated bandwidth.
Another important consideration in the design of spread spectrum systems is susceptibility to phase noise. In traditional multidimensional QPSK spread spectrum systems, the two dimensional data signals corresponding to in-phase (I) and quadrature (Q) components of the QPSK signal are each spread independently by separate spreading codes. Such independent spreading of the I and Q components means the combined complex signal does not necessarily represent each data symbol using antipodal signaling. This yields a modulation scheme characterized by degraded phase noise sensitivity as compared to antipodal modulation schemes.
U.S. Pat. No. 5,020,075 presents a “chip insertion” technique that utilizes a spread spectrum pseudo-random (PN) sequence in providing input to a minimum-shift-keying (MSK) modulator. In this patent, the PN sequence of chips is periodically interrupted with chips that represent symbols from a differentially encoded stream of data symbols. The resulting pre-modulation chip stream is presented to a minimum-shift-keying (MSK) modulator. To achieve Gaussian minimum-shift-keying (GMSK) modulation, the technique replaces the MSK modulator with a Gaussian filtered MSK modulator. However, this “chip insertion” technique is prone to significant performance degradations. For example, in “Performance of DS/GMSK/PSK Modem Using Four-Phase Correlator”, Yano, et. al, IEEE Spread Spectrum Techniques and Applications Proceedings, 1996, pgs. 249-253, the performance of a QPSK chip insertion system is shown to be about 2 dB away from theoretical performance bounds.
Improvements to current design that address these and other shortcomings would undoubtedly advance spread spectrum technology toward its full potential as a powerful communications methodology.